The grid array antenna was first proposed by Kraus in 1964. Since then, there have been some studies conducted but all were at relatively low microwave frequencies. FIG. 1 shows the basic grid arrangement. It consists of rectangular meshes of microstrip lines on a dielectric substrate backed by a metallic ground plane and fed by a metal via through an aperture on the ground plane. Depending on the electrical length of the sides of the meshes, the grid array antenna may be resonant or non-resonant.
For a resonant grid array antenna, the sides of the meshes should be one wavelength by a half-wavelength in the dielectric, and the instantaneous currents would be out of phase on the long sides of the meshes and in phase on the short sides of the meshes, respectively. As a result, the long sides of the meshes behave as microstrip line elements and the short sides act as both radiating and microstrip line elements. The short sides will produce the main lobe of radiation in the boresight direction.
For a non-resonant grid array antenna, the length of the short side of the meshes can be slightly more than one-third wavelength and the length of the long side of the meshes should be two times longer but three times shorter than the length of the short side of the meshes in the dielectric. Assuming that it is fed from one end, the currents in the short sides of the meshes follow a phase progression producing the maximum radiation in a backward angle-fire direction.
FIG. 2 shows the method of amplitude control through control of microstrip line impedances (or microstrip line widths) to lower the first sidelobe.
The grid array antenna has caught considerable attention since the middle of 1990s. FIG. 3(a) to (c) show the proposed miniaturized grid array antenna by:
(a) “meandering” the long sides of the meshes;
(b) dual-linearly-polarized grid array antenna by crossing the meshes; and
(c) a circularly-polarized grid array antenna by modifying the short sides of the meshes.
In addition, there has been developed a double-layer grid-array antenna. It consists of upper and lower grid array antennas, each being fed from its center terminal to radiate linearly-polarized waves. The upper and lower grid array antennas have the same configuration parameters. The orientation of the lower grid array antenna is rotated by 90° with respect to that of the upper grid array antenna. This perpendicular arrangement provides high isolation at both the center feeding terminals and results in one antenna radiating horizontally-polarized waves and the other antenna radiating vertically-polarized waves.
A cross-mesh array antenna is shown in FIG. 4. The radiation of circularly-polarized waves results from adding a layer of c-figured elements above the cross-mesh array antenna or feeding it at four terminals with signals of correct phase differences. The feeding terminals are shown in FIG. 4(b).
In the past, grid array antennas have been excited for single-ended signals. They may also be excited for differential signals. FIG. 5 illustrates a differential feeding scheme. One vertical (radiating) side of the center mesh is cut open with one end connected to the positive signal and the other end to the negative signal.
Typical antennas for millimeter wavelength signals are reflector, lens, and horn antennas. Reflector antenna technology has achieved the highest level of development for high gain applications. Lens antennas are a second high gain technology; while horn antennas limit gain to about 30 dBi due to construction limitations. Although these antennas all have a high gain, they are not suitable for commercial mm-wave radios because they are expensive, bulky, heavy and, more importantly, they cannot be integrated with solid-state devices. Printed, deposited or etched antenna arrays are used for mm-wave radio systems.
It has been proposed to use linearly-polarized mm-wave 60-GHz antenna arrays constructed on multilayer LTCC substrates. These antenna arrays use 4×4 microstrip patch radiating elements fed by a quarter-wavelength matched T-junction network and a Wilkinson power divider network, respectively. The measured results indicate that the antenna array fed by the matched T-junction network performs better than that fed by the Wilkinson power divider. The measured impedance bandwidths are 9.5% and 5.8% and maximum gains are 18.2 dBi and 15.7 dBi, respectively, for the antenna arrays with and without an embedded cavity.
Some antenna arrays have achieved wide bandwidth by three major technologies: original antenna element, laminated waveguide and design method to adjust axial ratio of circular polarization. The antenna element has laminated resonator structure formed by filled via-holes and conductive pattern, which generate wide bandwidth characteristics. Measurement results show that the array of 6×8 radiating elements has a sidelobe level less than −15 dB, gain variation less than 1 dB around 19 dBi and axial ratio less than 3 dB over a bandwidth more than 4 GHz.
Due to the selection of a microstrip patch and a slot as radiating elements, available antenna arrays require complex feeding networks, sophisticated process techniques, and additional embedded cavities to achieve the required performance. Also, available antenna arrays, if intended to be connected with differential radios, will require a feeding network that would become even more complex. Differential radios are more dominant than single-ended radios in highly-integrated mm-wave radios. Furthermore, the available antenna arrays provide an antenna function to the millimeter-wave radio devices. Hence, one can conclude that the available antenna arrays are yet not suitable for highly-integrated mm-wave 60-GHz radios because of their high cost and lower functionality.
It is known that for a resonant grid array antenna the instantaneous currents should be in phase on the short sides of the meshes. As such, the phasing of the radiating elements (short sides of the meshes) is critical. FIG. 8 shows instantaneous current distribution on the grid array antenna at 60-GHz. It is evident from the figure that the phase synchronism is only realized for the radiating elements between the two bars of dashed lines. Hence, the conventional grid array antenna will not perform well at mm-wave frequencies. Phase compensation schemes must be devised for mm-wave grid array antennas.